feat: add json logging

[icco]

Requirements

• I have added test coverage for new or changed functionality
• I have followed the repository's pull request submission guidelines
• I have validated my changes against all supported platform versions

Related issues

#562

Describe the solution you've provided

I tried not to modify the existing logging solution, and just added a simple switch to choose what logger you use.

In a perfect world I'd migrate this to zerolog or similar to add structured fields to logging calls.

Describe alternatives you've considered

See previous.

Additional context

Add any other context about the pull request here.

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Note

Adds structured logging support and configuration.

• New LOG_FORMAT env var and [Main] logFormat option (text default, json for structured logs)
• Introduces LogFormat/OptLogFormat types and parsing/validation
• Implements JSONLogger and ApplyLogFormat; default loggers honor LOG_FORMAT and Relay applies config-selected format
• Updates docs (configuration.md, logging.md) with format details and examples
• Adds unit tests for format parsing, default selection, JSON logger behavior

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